Internet-Draft Conveying a CSR in an SZTP Request July 2021
Watsen, et al. Expires 8 January 2022 [Page]
Workgroup:
NETCONF Working Group
Internet-Draft:
draft-ietf-netconf-sztp-csr-05
Updates:
8572 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
K. Watsen
Watsen Networks
R. Housley
Vigil Security, LLC
S. Turner
sn3rd

Conveying a Certificate Signing Request (CSR) in a Secure Zero Touch Provisioning (SZTP) Bootstrapping Request

Abstract

This draft extends the "get-bootstrapping-data" RPC defined in RFC 8572 to include an optional certificate signing request (CSR), enabling a bootstrapping device to additionally obtain an identity certificate (e.g., an LDevID, from IEEE 802.1AR) as part of the "onboarding information" response provided in the RPC-reply.

Editorial Note (To be removed by RFC Editor)

This draft contains many placeholder values that need to be replaced with finalized values at the time of publication. This note summarizes all of the substitutions that are needed. No other RFC Editor instructions are specified elsewhere in this document.

Artwork in this document contains shorthand references to drafts in progress. Please apply the following replacements:

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Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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▲

Table of Contents

1. Introduction

1.1. Overview

This draft extends the "get-bootstrapping-data" RPC defined in [RFC8572] to include an optional certificate signing request (CSR) [RFC2986], enabling a bootstrapping device to additionally obtain an identity certificate (e.g., an LDevID [Std-802.1AR-2018]) as part of the "onboarding information" response provided in the RPC-reply.

The ability to provision an identity certificate that is purpose-built for a production environment during the bootstrapping process removes reliance on the manufacturer CA, and it also enables the bootstraped device to join the production environment with an appropriate identity and other attributes in its LDevID certificate.

1.2. Terminology

This document uses the following terms from [RFC8572]:

  • Bootstrap Server
  • Bootstrapping Data
  • Conveyed Information
  • Device
  • Manufacturer
  • Onboarding Information
  • Signed Data

This document defines the following new terms:

SZTP-client
The term "SZTP-client" refers to a "device" that is using a "bootstrap server" as a source of "bootstrapping data".
SZTP-server
The term "SZTP-server" is an alternative term for "bootstrap server" that is symmetric with the "SZTP-client" term.

1.3. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

1.4. Conventions

Various examples used in this document use a placeholder value for binary data that has been base64 encoded (e.g., "BASE64VALUE="). This placeholder value is used as real base64 encoded structures are often many lines long and hence distracting to the example being presented.

2. The "ietf-sztp-csr" Module

This section defines a YANG 1.1 [RFC7950] module that augments the "ietf-sztp-bootstrap-server" module defined in [RFC8572] and defines a YANG "structure".

The augmentation adds two nodes ("csr-support" and "csr") to the "input" parameter of the "get-bootstrapping-data" RPC defined in [RFC8572].

The YANG structure, "request-info", defines data returned in the "error-info" node defined in Section 7.1 of [RFC8040].

2.1. Data Model Overview

The following tree diagram [RFC8340] illustrates the "ietf-sztp-csr" module. The diagram shows the definition of an augmentation adding descendant nodes "csr-support" and "csr" and the definition of a structure called "request-info".

In the order of their intended use:

module: ietf-sztp-csr

  augment /sztp-svr:get-bootstrapping-data/sztp-svr:input:
    +---w (msg-type)?
       +--:(csr-support)
       |  +---w csr-support!
       |     +---w key-generation!
       |     |  +---w supported-algorithms
       |     |     +---w algorithm-identifier*   binary
       |     +---w csr-generation
       |        +---w supported-formats
       |           +---w format-identifier*   identityref
       +--:(csr)
          +---w csr!
             +---w (request-type)
                +--:(p10)
                |  +---w p10?   ct:csr
                +--:(cmc)
                |  +---w cmc?   binary
                +--:(cmp)
                   +---w cmp?   binary

  structure: request-info
     +--ro key-generation!
     |  +--ro selected-algorithm
     |     +--ro algorithm-identifier    binary
     +--ro csr-generation
     |  +--ro selected-format
     |     +--ro format-identifier    identityref
     +--ro cert-req-info?    ietf-crypto-types:csr-info

To further illustrate how the augmentation and structure defined by the "ietf-sztp-csr" module are used, below are two additional tree diagrams showing these nodes placed where they are used.

The following tree diagram [RFC8340] illustrates SZTP's "get-bootstrapping-data" RPC with the augmentation in place.

=============== NOTE: '\' line wrapping per RFC 8792 ================

module: ietf-sztp-bootstrap-server

  rpcs:
    +---x get-bootstrapping-data
       +---w input
       |  +---w signed-data-preferred?        empty
       |  +---w hw-model?                     string
       |  +---w os-name?                      string
       |  +---w os-version?                   string
       |  +---w nonce?                        binary
       |  +---w (sztp-csr:msg-type)?
       |     +--:(sztp-csr:csr-support)
       |     |  +---w sztp-csr:csr-support!
       |     |     +---w sztp-csr:key-generation!
       |     |     |  +---w sztp-csr:supported-algorithms
       |     |     |     +---w sztp-csr:algorithm-identifier*   bina\
ry
       |     |     +---w sztp-csr:csr-generation
       |     |        +---w sztp-csr:supported-formats
       |     |           +---w sztp-csr:format-identifier*   identit\
yref
       |     +--:(sztp-csr:csr)
       |        +---w sztp-csr:csr!
       |           +---w (sztp-csr:request-type)
       |              +--:(sztp-csr:p10)
       |              |  +---w sztp-csr:p10?   ct:csr
       |              +--:(sztp-csr:cmc)
       |              |  +---w sztp-csr:cmc?   binary
       |              +--:(sztp-csr:cmp)
       |                 +---w sztp-csr:cmp?   binary
       +--ro output
          +--ro reporting-level?    enumeration {onboarding-server}?
          +--ro conveyed-information    cms
          +--ro owner-certificate?      cms
          +--ro ownership-voucher?      cms

The following tree diagram [RFC8340] illustrates RESTCONF's "errors" RPC-reply message with the "request-info" structure in place.

module: ietf-restconf
  +--ro errors
     +--ro error* []
        +--ro error-type       enumeration
        +--ro error-tag        string
        +--ro error-app-tag?   string
        +--ro error-path?      instance-identifier
        +--ro error-message?   string
        +--ro error-info
           +--ro request-info
              +--ro key-generation!
              |  +--ro selected-algorithm
              |     +--ro algorithm-identifier    binary
              +--ro csr-generation
              |  +--ro selected-format
              |     +--ro format-identifier    identityref
              +--ro cert-req-info?    ct:csr-info

2.2. Example Usage

An SZTP-client implementing this specification would signal to the bootstrap server its willingness to generate a CSR by including the "csr-support" node in its "get-bootstrapping-data" RPC, as illustrated below.

REQUEST

=============== NOTE: '\' line wrapping per RFC 8792 ================

POST /restconf/operations/ietf-sztp-bootstrap-server:get-bootstrappi\
ng-data HTTP/1.1
HOST: example.com
Content-Type: application/yang.data+json

{
  "ietf-sztp-bootstrap-server:input" : {
    "hw-model": "model-x",
    "os-name": "vendor-os",
    "os-version": "17.3R2.1",
    "nonce": "extralongbase64encodedvalue=",
    "ietf-sztp-csr:csr-support": {
      "key-generation": {
        "supported-algorithms": {
          "algorithm-identifier": [
            "BASE64VALUE1",
            "BASE64VALUE2",
            "BASE64VALUE3"
          ]
        }
      },
      "csr-generation": {
        "supported-formats": {
          "format-identifier": [
            "ietf-sztp-csr:p10",
            "ietf-sztp-csr:cmc",
            "ietf-sztp-csr:cmp"
          ]
        }
      }
    }
  }
}

Assuming the SZTP-server wishes to prompt the SZTP-client to provide a CSR, then it would respond with an HTTP 400 Bad Request error code:

RESPONSE

HTTP/1.1 400 Bad Request
Date: Sat, 31 Oct 2015 17:02:40 GMT
Server: example-server
Content-Type: application/yang.data+json

{
  "ietf-restconf:errors" : {
    "error" : [
      {
        "error-type": "application",
        "error-tag": "missing-attribute",
        "error-message": "Missing input parameter",
        "error-info": {
          "ietf-sztp-csr:request-info": {
            "key-generation": {
              "selected-algorithm": {
                "algorithm-identifier": "BASE64VALUE="
              }
            },
            "csr-generation": {
              "selected-format": {
                "format-identifier": "ietf-sztp-csr:cmc"
              }
            },
            "cert-req-info": "BASE64VALUE="
          }
        }
      }
    ]
  }
}

Upon being prompted to provide a CSR, the SZTP-client would POST another "get-bootstrapping-data" request, but this time including the "csr" node to convey its CSR to the SZTP-server:

REQUEST

=============== NOTE: '\' line wrapping per RFC 8792 ================

POST /restconf/operations/ietf-sztp-bootstrap-server:get-bootstrappi\
ng-data HTTP/1.1
HOST: example.com
Content-Type: application/yang.data+json

{
  "ietf-sztp-bootstrap-server:input" : {
    "hw-model": "model-x",
    "os-name": "vendor-os",
    "os-version": "17.3R2.1",
    "nonce": "extralongbase64encodedvalue=",
    "ietf-sztp-csr:csr": {
      "p10": "BASE64VALUE="
    }
  }
}

The SZTP-server responds with "onboarding-information" (encoded inside the "conveyed-information" node) containing a signed identity certificate for the CSR provided by the SZTP-client:

RESPONSE

HTTP/1.1 200 OK
Date: Sat, 31 Oct 2015 17:02:40 GMT
Server: example-server
Content-Type: application/yang.data+json

{
  "ietf-sztp-bootstrap-server:output" : {
    "reporting-level": "verbose",
    "conveyed-information": "BASE64VALUE="
  }
}

How the signed certificate is conveyed inside the onboarding information is outside the scope of this document. Some implementations may choose to convey it inside a script (e.g., SZTP's "pre-configuration-script"), while other implementations may choose to convey it inside the SZTP "configuration" node. SZTP onboarding information is described in Section 2.2 of [RFC8572].

Following are two examples of conveying the signed certificate inside the "configuration" node. Both examples assume that the SZTP-client understands the "ietf-keystore" module defined in [I-D.ietf-netconf-keystore].

This first example illustrates the case where the signed certificate is for the same asymmetric key used by the SZTP-client's manufacturer-generated identity certificate (e.g., an IDevID, from [Std-802.1AR-2018]). As such, the configuration needs to associate the newly signed certificate with the existing asymmetric key:

=============== NOTE: '\' line wrapping per RFC 8792 ================

{
  "ietf-keystore:keystore": {
    "asymmetric-keys": {
      "asymmetric-key": [
        {
          "name": "Manufacturer-Generated Hidden Key",
          "public-key-format": "ietf-crypto-types:subject-public-key\
-info-format",
          "public-key": "BASE64VALUE=",
          "hidden-private-key": [null],
          "certificates": {
            "certificate": [
              {
                "name": "Manufacturer-Generated IDevID Cert",
                "cert-data": "BASE64VALUE="
              },
              {
                "name": "Newly-Generated LDevID Cert",
                "cert-data": "BASE64VALUE="
              }
            ]
          }
        }
      ]
    }
  }
}

This second example illustrates the case where the signed certificate is for a newly generated asymmetric key. As such, the configuration needs to associate the newly signed certificate with the newly generated asymmetric key:

=============== NOTE: '\' line wrapping per RFC 8792 ================

{
  "ietf-keystore:keystore": {
    "asymmetric-keys": {
      "asymmetric-key": [
        {
          "name": "Manufacturer-Generated Hidden Key",
          "public-key-format": "ietf-crypto-types:subject-public-key\
-info-format",
          "public-key": "BASE64VALUE=",
          "hidden-private-key": [null],
          "certificates": {
            "certificate": [
              {
                "name": "Manufacturer-Generated IDevID Cert",
                "cert-data": "BASE64VALUE="
              }
            ]
          }
        },
        {
          "name": "Newly-Generated Hidden Key",
          "public-key-format": "ietf-crypto-types:subject-public-key\
-info-format",
          "public-key": "BASE64VALUE=",
          "hidden-private-key": [null],
          "certificates": {
            "certificate": [
              {
                "name": "Newly-Generated LDevID Cert",
                "cert-data": "BASE64VALUE="
              }
            ]
          }
        }
      ]
    }
  }
}

In addition to configuring the signed certificate, it is often necessary to also configure the Issuer's signing certificate so that the device (i.e., STZP-client) can authenticate certificates presented by peer devices signed by the same issuer as its own. While outside the scope of this document, one way to do this would be to use the "ietf-truststore" module defined in [I-D.ietf-netconf-trust-anchors].

2.3. YANG Module

This module augments an RPC defined in [RFC8572], uses a data type defined in [I-D.ietf-netconf-crypto-types], has normative references to [RFC2986] and [ITU.X690.2015], and an informative reference to [Std-802.1AR-2018].

<CODE BEGINS> file "ietf-sztp-csr@2021-07-07.yang"

module ietf-sztp-csr {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-sztp-csr";
  prefix sztp-csr;

  import ietf-sztp-bootstrap-server {
    prefix sztp-svr;
    reference
      "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
  }

  import ietf-yang-structure-ext {
    prefix sx;
    reference
      "RFC 8791: YANG Data Structure Extensions";
  }

  import ietf-crypto-types {
    prefix ct;
    reference
      "RFC AAAA: YANG Data Types and Groupings for Cryptography";
  }

  organization
    "IETF NETCONF (Network Configuration) Working Group";

  contact
    "WG Web:   http://tools.ietf.org/wg/netconf
     WG List:  <mailto:netconf@ietf.org>
     Authors:  Kent Watsen <mailto:kent+ietf@watsen.net>
               Russ Housley <mailto:housley@vigilsec.com>
               Sean Turner <mailto:sean@sn3rd.com>";

  description
    "This module augments the 'get-bootstrapping-data' RPC,
     defined in the 'ietf-sztp-bootstrap-server' module from
     SZTP (RFC 8572), enabling the SZTP-client to obtain a
     signed identity certificate (e.g., an LDevID from IEEE
     802.1AR) as part of the SZTP onboarding information
     response.

     Copyright (c) 2020 IETF Trust and the persons identified
     as authors of the code. All rights reserved.

     Redistribution and use in source and binary forms, with
     or without modification, is permitted pursuant to, and
     subject to the license terms contained in, the Simplified
     BSD License set forth in Section 4.c of the IETF Trust's
     Legal Provisions Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXXX
     (https://www.rfc-editor.org/info/rfcXXXX); see the RFC
     itself for full legal notices.

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
     'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
     'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this
     document are to be interpreted as described in BCP 14
     (RFC 2119) (RFC 8174) when, and only when, they appear
     in all capitals, as shown here.";

  revision 2021-07-07 {
    description
      "Initial version";
    reference
      "RFC XXXX: Conveying a Certificate Signing Request (CSR)
                 in a Secure Zero Touch Provisioning (SZTP)
                 Bootstrapping Request";
  }

  identity certificate-request-format {
    description
      "A base identity for the request formats supported
       by the SZTP-client.

       Additional derived identities MAY be defined by
       future efforts.";
  }

  identity p10 {
    base certificate-request-format;
    description
      "Indicates that the SZTP-client supports generating
       requests using the 'CertificationRequest' structure
       defined in RFC 2986.";
    reference
      "RFC 2986: PKCS #10: Certification Request Syntax
                 Specification Version 1.7";
  }

  identity cmc {
    base certificate-request-format;
    description
      "Indicates that the SZTP-client supports generating
       requests using a constrained version of the 'Full
       PKI Request' structure defined in RFC 5272.";
    reference
      "RFC 5272: Certificate Management over CMS (CMC)";
  }

  identity cmp {
    base certificate-request-format;
    description
      "Indicates that the SZTP-client supports generating
       requests that contain a PKCS#10 Certificate Signing
       Request (p10cr), as defined in RFC 2986, encapsulated
       in a Nested Message Content (nested), as defined in
       RFC 4210.";
    reference
      "RFC 2986: PKCS #10: Certification Request Syntax
                 Specification Version 1.7
       RFC 4210: Internet X.509 Public Key Infrastructure
                 Certificate Management Protocol (CMP)";
  }

  // Protocol-accessible nodes

  augment "/sztp-svr:get-bootstrapping-data/sztp-svr:input" {
    description
      "This augmentation adds the 'csr-support' and 'csr' nodes to
       the SZTP (RFC 8572) 'get-bootstrapping-data' request message,
       enabling the SZTP-client to obtain an identity certificate
       (e.g., an LDevID from IEEE 802.1AR) as part of the onboarding
       information response provided by the SZTP-server.

       The 'csr-support' node enables the SZTP-client to indicate
       that it supports generating certificate signing requests
       (CSRs), and to provide details around the CSRs it is able
       to generate.

       The 'csr' node enables the SZTP-client to relay a CSR to
       the SZTP-server.";
    reference
      "IEEE 802.1AR: IEEE Standard for Local and metropolitan
                     area networks - Secure Device Identity
       RFC 8572: Secure Zero Touch Provisioning (SZTP)";
    choice msg-type {
      description
        "Only a 'csr-support' or a 'csr' message may be sent.";
      container csr-support {
        presence
          "Indicates that the SZTP-client is capable of sending
           CSRs.";
        description
          "The 'csr-support' node enables the SZTP-client to
           indicate that it supports generating certificate
           signing requests (CSRs), and provide details around
           the CSRs it is able to generate.

           When present, the SZTP-server MAY respond with the HTTP
           code 400 Bad Request with an 'ietf-restconf:errors'
           document having the 'error-tag' value 'missing-attribute'
           and the 'error-info' node containing the 'request-info'
           structure described in this module.";
        container key-generation {
          presence
            "Indicates that the SZTP-client is capable of
             generating a new asymmetric key pair.

             If this node is not present, the SZTP-server MAY
             request a CSR using the asymmetric key associated
             with the device's existing identity certificate
             (e.g., an IDevID from IEEE 802.1AR).";
          description
            "Specifies details for the SZTP-client's ability to
             generate a new asymmetric key pair.";
          container supported-algorithms {
            description
              "A list of public key algorithms supported by the
               SZTP-client for generating a new key.";
            leaf-list algorithm-identifier {
              type binary;
              min-elements 1;
              description
                "An AlgorithmIdentifier, as defined in RFC 2986,
                 encoded using ASN.1 distinguished encoding rules
                 (DER), as specified in ITU-T X.690.";
              reference
                "RFC 2986: PKCS #10: Certification Request Syntax
                           Specification Version 1.7
                 ITU-T X.690:
                   Information technology - ASN.1 encoding rules:
                   Specification of Basic Encoding Rules (BER),
                   Canonical Encoding Rules (CER) and Distinguished
                   Encoding Rules (DER).";
            }
          }
        }
        container csr-generation {
          description
            "Specifies details for the SZTP-client's ability to
             generate a certificate signing requests.";
          container supported-formats {
            description
              "A list of certificate request formats supported
               by the SZTP-client for generating a new key.";
            leaf-list format-identifier {
              type identityref {
                base certificate-request-format;
              }
              min-elements 1;
              description
                "A certificate request format supported by the
                 SZTP-client.";
            }
          }
        }
      }
      container csr {
        presence "Indicates that the SZTP-client has sent a CSR.";
        description
          "The 'csr' node enables the SZTP-client to convey
           a certificate signing request, using the encoding
           format selected by the SZTP-server's 'request-info'
           response to the SZTP-client's previously sent
           'get-bootstrapping-data' request containing the
           'csr-support' node.

           When present, the SZTP-server SHOULD respond with
           an SZTP onboarding information message containing
           a signed certificate for the conveyed CSR.  The
           SZTP-server MAY alternatively respond with another
           HTTP error containing another 'request-info', in
           which case the SZTP-client MUST invalidate the CSR
           sent in this node.";
        choice request-type {
          mandatory true;
          description
            "A choice amongst certificate signing request formats.

             Additional formats MAY be augmented into this 'choice'
             statement by future efforts.";
          case p10 {
            leaf p10 {
              type ct:csr;
              description
                "A CertificationRequest structure, per RFC 2986.
                 Please see 'csr' in RFC AAAA for encoding details.";
              reference
                "RFC 2986: PKCS #10: Certification
                           Request Syntax Specification
                 RFC AAAA: YANG Data Types and Groupings
                           for Cryptography";
            }
          }
          case cmc {
            leaf cmc {
              type binary;
              description
                "A constrained version of the 'Full PKI Request'
                 message defined in RFC 5272, encoded using ASN.1
                 distinguished encoding rules (DER), as specified
                 in ITU-T X.690.

                 For asymmetric key-based origin authentication
                 of a CSR based on the IDevID's private key for the
                 associated IDevID's public key, the PKIData contains
                 one reqSequence element and no controlSequence,
                 cmsSequence, or otherMsgSequence elements. The
                 reqSequence is the TaggedRequest and it is the tcr
                 CHOICE. The tcr is the TaggedCertificationRequest
                 and it a bodyPartId and the certificateRequest
                 elements. The certificateRequest is signed with
                 the IDevID's private key.

                 For asymmetric key-based origin authentication
                 based on the IDevID's private key that encapsulates
                 a CSR signed by the LDevID's private key, the
                 PKIData contains one cmsSequence element and no
                 controlSequence, reqSequence, or otherMsgSequence
                 elements. The cmsSequence is the TaggedContentInfo
                 and it includes a bodyPartID element and a
                 contentInfo. The contentInfo is a SignedData
                 encapsulating a PKIData with one reqSequence
                 element and no controlSequence, cmsSequence, or
                 otherMsgSequence elements. The reqSequence is
                 the TaggedRequest and it is the tcr CHOICE. The
                 tcr is the TaggedCertificationRequest and it a
                 bodyPartId and the certificateRequest elements.
                 The certificateRequest is signed with the LDevID's
                 private key.

                 For shared secret-based origin authentication of
                 a CSR signed by the LDevID's private key, the
                 PKIData contains one cmsSequence element and no
                 controlSequence, reqSequence, or otherMsgSequence
                 elements. The cmsSequence is the TaggedContentInfo
                 and it includes a bodyPartID element and a
                 contentInfo. The contentInfo is an AuthenticatedData
                 encapsulating a PKIData with one reqSequence
                 element and no controlSequence, cmsSequence, or
                 otherMsgSequence elements. The reqSequence is the
                 TaggedRequest and it is the tcr CHOICE. The tcr
                 is the TaggedCertificationRequest and it a
                 bodyPartId and the certificateRequest elements.
                 The certificateRequest is signed with the LDevID's
                 private key.";
              reference
                "RFC 5272: Certificate Management over CMS (CMC)
                 ITU-T X.690:
                   Information technology - ASN.1 encoding rules:
                   Specification of Basic Encoding Rules (BER),
                   Canonical Encoding Rules (CER) and Distinguished
                   Encoding Rules (DER).";
            }
          }
          case cmp {
            leaf cmp {
              type binary;
              description
                "A PKIMessage structure, as defined in RFC 4210,
                 encoded using ASN.1 distinguished encoding rules
                 (DER), as specified in ITU-T X.690.

                 The PKIMessage structure contains a PKCS#10
                 Certificate Signing Request (p10cr), as defined in
                 RFC 2986, encapsulated in a Nested Message Content
                 (nested) structure, as defined in RFC 4210.

                 For asymmetric key-based origin authentication of
                 a CSR based on the IDevID's private key for the
                 associated IDevID's public key, PKIMessages contains
                 one PKIMessage with one body element, a header
                 element that is an empty sequence, and no protection
                 or extraCerts elements. The body element contains a
                 p10cr CHOICE.

                 For asymmetric key-based origin authentication based
                 on the IDevID's private key that encapsulates a CSR
                 signed by the LDevID's private key, PKIMessages
                 contains one PKIMessage with one header element,
                 one body element, one protection element, and one
                 extraCerts element.  The header element contains
                 pvno, sender, recipient, and protectionAlg elements
                 and no other elements. The body element contains the
                 nested CHOICE. The nested element's PKIMessages
                 contains one PKIMessage with one body element, one
                 header element that is an empty sequence, and no
                 protection or extraCerts elements. The nested
                 element's body element contains a p10cr CHOICE. The
                 protection element contains the digital signature
                 generated with the IDevID's private key. The
                 extraCerts element contains the IDevID certificate.

                 For shared secret-based origin authentication of a
                 CSR signed by the LDevID's private key, PKIMessages
                 contains one PKIMessage with one header element,
                 one body element, one protection element, and no
                 extraCerts element. The header element contains
                 pvno, sender, recipient, and protectionAlg elements
                 and no other elements. The body element contains
                 the nested CHOICE. The nested element's PKIMessages
                 contains one PKIMessage with one body element, one
                 header element that is an empty sequence, and no
                 protection or extraCerts elements. The body element
                 contains a p10cr CHOICE. The protection element
                 contains the MAC value generated with the shared
                 secret.";
              reference
                "RFC 2986:
                   PKCS #10: Certification Request Syntax
                   Specification Version 1.7
                 RFC 4210:
                   Internet X.509 Public Key Infrastructure
                   Certificate Management Protocol (CMP)
                 ITU-T X.690:
                   Information technology - ASN.1 encoding rules:
                   Specification of Basic Encoding Rules (BER),
                   Canonical Encoding Rules (CER) and Distinguished
                   Encoding Rules (DER).";
            }
          }
        }
      }
    }
  }

  sx:structure request-info {
    container key-generation {
      presence
        "Indicates that the SZTP-client is to generate a new
         asymmetric key.  If missing, then the SZTP-client
         MUST reuse the key associated with its existing
         identity certificate (e.g., IDevID).

         This leaf MUST only appear if the SZTP-clients
         'csr-support' included the 'key-generation' node.";
      description
        "A YANG data structure, per RFC 8791, that specifies
         details for the key that the SZTP-client is to
         generate.";
      reference
        "RFC 8791: YANG Data Structure Extensions";
      container selected-algorithm {
        description
          "The key algorithm selected by the SZTP-server. The
           algorithm MUST be one of the algorithms specified
           by the 'supported-algorithms' node in the
           SZTP-client's request message.";
        leaf algorithm-identifier {
          type binary;
          mandatory true;
          description
            "An AlgorithmIdentifier, as defined in RFC 2986,
             encoded using ASN.1 distinguished encoding rules
             (DER), as specified in ITU-T X.690.";
          reference
            "RFC 2986: PKCS #10: Certification Request Syntax
                       Specification Version 1.7
             ITU-T X.690:
               Information technology - ASN.1 encoding rules:
               Specification of Basic Encoding Rules (BER),
               Canonical Encoding Rules (CER) and Distinguished
               Encoding Rules (DER).";
        }
      }
    }
    container csr-generation {
      description
        "Specifies details for the CSR that the SZTP-client
         is to generate.";
      container selected-format {
        description
          "The CSR format selected by the SZTP-server. The
           format MUST be one of the formats specified by
           the 'supported-formats' node in the SZTP-client's
           request message.";
        leaf format-identifier {
          type identityref {
            base certificate-request-format;
          }
          mandatory true;
          description
            "A certificate request format to be used by the
             SZTP-client.";
        }
      }
    }
    leaf cert-req-info {
      type ct:csr-info;
      description
        "A CertificationRequestInfo structure, as defined in
         RFC 2986, and modeled via a 'typedef' statement by
         RFC AAAA.

         Enables the SZTP-server to provide a fully-populated
         CertificationRequestInfo structure that the SZTP-client
         only needs to sign in order to generate the complete
         'CertificationRequest' structure to send to SZTP-server
         in its next 'get-bootstrapping-data' request message.

         When provided, the SZTP-client SHOULD use this
         structure to generate its CSR; failure to do so MAY
         result in a 400 Bad Request response containing
         another 'request-info' structure.

         When not provided, the SZTP-client SHOULD generate a
         CSR using the same structure defined in its existing
         identity certificate (e.g., IDevID).

         It is an error if the 'AlgorithmIdentifier' field
         contained inside the 'SubjectPublicKeyInfo' field
         does not match the algorithm identified by the
         'selected-algorithm' node.";
      reference
        "RFC 2986:
           PKCS #10: Certification Request Syntax Specification
         RFC AAAA:
           YANG Data Types and Groupings for Cryptography";
    }
  }
}

<CODE ENDS>

3. Security Considerations

This document builds on top of the solution presented in [RFC8572] and therefore all the Security Considerations discussed in RFC 8572 apply here as well.

3.1. SZTP-Client Considerations

3.1.1. Ensuring the Integrity of Asymmetric Private Keys

The private key the SZTP-client uses for the dynamically-generated identity certificate MUST be protected from inadvertent disclosure in order to prevent identity fraud.

The security of this private key is essential in order to ensure the associated identity certificate can be used as a root of trust.

It is RECOMMENDED that devices are manufactured with an HSM (hardware security module), such as a TPM (trusted platform module), to generate and forever contain the private key within the security perimeter of the HSM. In such cases, the private key, and its associated certificates, MAY have long validity periods.

In cases where the device does not possess an HSM, or otherwise is unable to use an HSM for the private key, it is RECOMMENDED to regenerate the private key (and associated identity certificates) periodically. Details for how to generate a new private key and associate a new identity certificate are outside the scope of this document.

3.1.2. Reuse of a Manufacturer-generated Private Key

It is RECOMMENDED that a new private key is generated for each CSR described in this document.

This private key SHOULD be protected as well as the built-in private key associated with the device's initial secure device identity certificate (e.g., the IDevID, from [Std-802.1AR-2018]).

In cases where it is not possible to generate a new private key that is protected as well as the built-in private key, it is RECOMMENDED to reuse the built-in private key rather than generate a new private key that is not as well protected.

3.1.3. Replay Attack Protection

This RFC enables an SZTP-client to announce an ability to generate a new key to use for its CSR.

When the SZTP-server responds with a request for the device to generate a new key, it is essential that the device actually generates a new key.

Generating a new key each time enables the random bytes used to create the key to also serve the dual-purpose of acting like a "nonce" used in other mechanisms to detect replay attacks.

When a fresh public/private key pair is generated for the request, confirmation to the SZTP-client that the response has not been replayed is enabled by the SZTP-client's fresh public key appearing in the signed certificate provided by the SZTP-server.

When a public/private key pair associated with the manufacturer-generated identity certificate (e.g., IDevID) is used for the request, there may not be confirmation to the SZTP-client that the response has not been replayed; however, the worst case result is a lost certificate that is associated to the private key known only to the SZTP-client.

3.1.4. Connecting to an Untrusted Bootstrap Server

[RFC8572] allows SZTP-clients to connect to untrusted SZTP-servers, by blindly authenticating the SZTP-server's TLS end-entity certificate.

As is discussed in Section 9.5 of [RFC8572], in such cases the SZTP-client MUST assert that the bootstrapping data returned is signed, if the SZTP-client is to trust it.

However, the HTTP error message used in this document cannot be signed data, as described in RFC 8572.

Therefore, the solution presented in this document cannot be used when the SZTP-client connects to an untrusted SZTP-server.

Consistent with the recommendation presented in Section 9.6 of [RFC8572], SZTP-clients SHOULD NOT pass the "csr-support" input parameter to an untrusted SZTP-server. SZTP-clients SHOULD pass instead the "signed-data-preferred" input parameter, as discussed in Appendix B of [RFC8572].

3.1.5. Selecting the Best Origin Authentication Mechanism

When generating a new key, it is important that the client be able to provide additional proof to the CA that it was the entity that generated the key.

All the certificate request formats defined in this document (e.g., CMC, CMP, etc.), not including a raw PKCS#10, support origin authentication.

These formats support origin authentication using both PKI and shared secret.

Typically, only one possible origin authentication mechanism can possibly be used but, in the case that the SZTP-client authenticates itself using both TLS-level (e.g., IDevID) and HTTP-level credentials (e.g., Basic), as is allowed by Section 5.3 of [RFC8572], then the SZTP-client may need to choose between the two options.

In the case that the SZTP-client must choose between the asymmetric key option versus a shared secret for origin authentication, it is RECOMMENDED that the SZTP-client choose using the asymmetric key option.

3.1.6. Clearing the Private Key and Associated Certificate

Unlike a manufacturer-generated identity certificate (e.g., IDevID), the deployment-generated identity certificate (e.g., LDevID) and the associated private key (assuming a new private key was generated for the purpose), are considered user data and SHOULD be cleared whenever the device is reset to its factory default state, such as by the "factory-reset" RPC defined in [I-D.ietf-netmod-factory-default].

3.2. SZTP-Server Considerations

3.2.1. Conveying Proof of Possession to a CA

When the bootstrapping device's manufacturer-generated private key (e.g., the IDevID key) is reused, a CA may validate that the CSR was signed by that key.

Both the CMP and CMC formats entail the bootstrapping device signing the request once with its (e.g., LDevID) key and then again with its (e.g., IDevID) key, which enables a downstream CA to be assured that the device possesses the public key being signed.

3.2.2. Supporting SZTP-Clients that don't trust the SZTP-Server

[RFC8572] allows SZTP-clients to connect to untrusted SZTP-servers, by blindly authenticating the SZTP-server's TLS end-entity certificate.

As is recommended in Section 3.1.4 in this document, in such cases, SZTP-clients SHOULD pass the "signed-data-preferred" input parameter.

The reciprocal of this statement is that SZTP-servers, wanting to support SZTP-clients that don't trust them, SHOULD support the "signed-data-preferred" input parameter, as discussed in Appendix B of [RFC8572].

3.2.3. YANG Module Considerations

The recommended format for documenting the Security Considerations for YANG modules is described in Section 3.7 of [RFC8407]. However, the module defined in this document only augments two input parameters into the "get-bootstrapping-data" RPC in [RFC8572], and therefore only needs to point to the relevant Security Considerations sections in that RFC.

  • Security considerations for the "get-bootstrapping-data" RPC are described in Section 9.16 of [RFC8572].
  • Security considerations for the "input" parameters passed inside the "get-bootstrapping-data" RPC are described in Section 9.6 of [RFC8572].

4. IANA Considerations

4.1. The "IETF XML" Registry

This document registers one URI in the "ns" subregistry of the IETF XML Registry [RFC3688] maintained at https://www.iana.org/assignments/xml-registry/xml-registry.xhtml#ns. Following the format in [RFC3688], the following registration is requested:

URI: urn:ietf:params:xml:ns:yang:ietf-sztp-csr
Registrant Contact: The NETCONF WG of the IETF.
XML: N/A, the requested URI is an XML namespace.

4.2. The "YANG Module Names" Registry

This document registers one YANG module in the YANG Module Names registry [RFC6020] maintained at https://www.iana.org/assignments/yang-parameters/yang-parameters.xhtml. Following the format defined in [RFC6020], the below registration is requested:

name:      ietf-sztp-csr
namespace: urn:ietf:params:xml:ns:yang:ietf-sztp-csr
prefix:    sztp-csr
reference: RFC XXXX

5. References

5.1. Normative References

[I-D.ietf-netconf-crypto-types]
Watsen, K., "YANG Data Types and Groupings for Cryptography", Work in Progress, Internet-Draft, draft-ietf-netconf-crypto-types-19, , <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-crypto-types-19>.
[ITU.X690.2015]
International Telecommunication Union, "Information Technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1, , <https://www.itu.int/rec/T-REC-X.690/>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC2986]
Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, DOI 10.17487/RFC2986, , <https://www.rfc-editor.org/info/rfc2986>.
[RFC3688]
Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, , <https://www.rfc-editor.org/info/rfc3688>.
[RFC6020]
Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, , <https://www.rfc-editor.org/info/rfc6020>.
[RFC7950]
Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, , <https://www.rfc-editor.org/info/rfc7950>.
[RFC8040]
Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, , <https://www.rfc-editor.org/info/rfc8040>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8572]
Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero Touch Provisioning (SZTP)", RFC 8572, DOI 10.17487/RFC8572, , <https://www.rfc-editor.org/info/rfc8572>.

5.2. Informative References

[I-D.ietf-netconf-keystore]
Watsen, K., "A YANG Data Model for a Keystore", Work in Progress, Internet-Draft, draft-ietf-netconf-keystore-21, , <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-keystore-21>.
[I-D.ietf-netconf-trust-anchors]
Watsen, K., "A YANG Data Model for a Truststore", Work in Progress, Internet-Draft, draft-ietf-netconf-trust-anchors-14, , <https://datatracker.ietf.org/doc/html/draft-ietf-netconf-trust-anchors-14>.
[I-D.ietf-netmod-factory-default]
Wu, Q., Lengyel, B., and Y. Niu, "A YANG Data Model for Factory Default Settings", Work in Progress, Internet-Draft, draft-ietf-netmod-factory-default-15, , <https://datatracker.ietf.org/doc/html/draft-ietf-netmod-factory-default-15>.
[RFC8340]
Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, , <https://www.rfc-editor.org/info/rfc8340>.
[RFC8407]
Bierman, A., "Guidelines for Authors and Reviewers of Documents Containing YANG Data Models", BCP 216, RFC 8407, DOI 10.17487/RFC8407, , <https://www.rfc-editor.org/info/rfc8407>.
[Std-802.1AR-2018]
Group, W. -. H. L. L. P. W., "IEEE Standard for Local and metropolitan area networks - Secure Device Identity", , <http://standards.ieee.org/findstds/standard/802.1AR-2018.html>.

Authors' Addresses

Kent Watsen
Watsen Networks
Russ Housley
Vigil Security, LLC
Sean Turner
sn3rd